A variety of fluid pressure-actuated components include a valve mechanism that is controllable to switch operating modes of the component by directing pressurized fluid to the component. For example, variable valve actuation systems are sometimes used to control the amount of lift of an engine valve and the associated flow of combustion gas into or out of an engine cylinder. Additionally, an engine pump may be operable in a high output mode and a low output mode, with a valve mechanism controlling pressurized fluid flow to the pump to control a switch between the two modes. It may be desirable to closely control the timing of a switch between modes.
Engines generally may have a timing cycle that is based on a four-stroke engine cycle. Variable valve actuation systems can be switched between modes only during a predetermined window of time. Shifting the variable valve actuation system outside of the timing window may result in a critical shift event, which is a shift in engine valve position during a point in the engine cycle when loading on the valve actuator switching component or on the engine valve is higher than the structure is designed to accommodate while switching. A critical shift event may result in damage to the valvetrain and/or other engine parts. Depending upon the engine design and engine speed, the timing window for shifting a variable valve actuation system may be confined to a very limited period of time, for example, 10 milliseconds.